Along with Merton Miller, he formulated the important Modigliani–Miller theorem in corporate finance (1958). This theorem demonstrated that under certain assumptions, the value of a firm is not affected by whether it is financed by equity (selling shares) or debt (borrowing money).

He was also the originator of the life-cycle hypothesis, which attempts to explain the level of saving in the economy. Modigliani proposed that consumers would aim for a stable level of consumption throughout their lifetime, for example by saving during their working years and spending during their retirement.

He has claimed that the Rational Expectations Hypothesis (REH) rests in his and Emile Grunberg's paper.[3] He has also used the concept Macro Rational Expectations Hypothesis (MREH).[4]

In 1962, he joined the faculty at MIT, achieving distinction as an Institute Professor, where he stayed until his death. In 1985 he received MIT's James R. Killian Faculty Achievement Award.[5]

In the 1990s he teamed up with Francis Vitagliano to work on a new credit card, and he also helped to oppose changes to a patent law that would be harmful to inventors.

He is the co-author of Rethinking Pension Reform (2009), Cambridge University Press, and along with Arun Muralidhar, critiqued the privatization model of Social Security reform proposed by the World Bank (in the 1990s) and President Bush in the early 2000s, and offered a better alternative to reform Social Security systems globally.